In ink-jet printing, tiny drops of ink fluid are projected directly onto an ink receptor surface without physical contact between the printing device and the receptor. The printing device stores the printing data electronically and controls a mechanism for ejecting the drops image-wise. Printing is accomplished by moving a print head across the paper or vice versa. Early patents on ink-jet printers include U.S. Pat. No. 3,739,393 (MEAD CORP), U.S. Pat. No. 3,805,273 (MEAD CORP) and U.S. Pat. No. 3,891,121 (MEAD CORP)
Ink compositions for ink-jet typically include following ingredients: dyes or pigments, water and/or organic solvents, humectants such as glycols, detergents, thickeners, polymeric binders, preservatives, etc. It will be readily understood that the optimal composition of such ink is dependent on the ink-jetting method used and on the nature of the ink-receiver to be printed. The ink compositions can be roughly divided in:                water-based, the drying mechanism involving absorption, penetration and evaporation;        oil-based, the drying involving absorption and penetration;        solvent-based, the drying primarily involving evaporation;        hot melt or phase change, in which the ink is liquid at the ejection temperature but solid at room temperature and wherein drying is replaced by solidification;        UV-curable, in which drying is replaced by polymerization.        
It will be readily understood that the first two types of ink compositions are more suitable for a receiving medium that is more or less absorptive, whereas hot melt inks and UV-curable inks are more suitable for non-absorbent ink-receivers.
Early patents on water-based inks include U.S. Pat. No. 3,903,034 (AB DICK CO), U.S. Pat. No. 3,889,269 (AGFA GEVAERT), U.S. Pat. No. 3,870,528 (IBM), U.S. Pat. No. 3,846,141 (AB DICK CO), U.S. Pat. No. 3,776,742 (MEAD CORP) and U.S. Pat. No. 3,705,043 (AB DICK CO). However, systems based on water-based inks suffer from a number of disadvantages such as: (a) their requiring water evaporation and therefore an extensive drying system, especially when printing speed is important; (b) the tendency of large printed areas to cockle, (c) sensitivity of images to wet and dry rubbing, (d) the tendency of low viscosity inks to dry at the tip of the orifice which can be avoided by the use of humectants, usually glycols, which increase the viscosity. The use of polar solvent-based inks can overcome some of the problems inherent in water-based inks, but results in other problems such as the possible generation of toxic or inflammable vapours. Therefore efforts were made to develop low-solvent ink compositions from which the concept of UV-curable ink compositions emerged.
U.S. Pat. No. 4,303,924 (MEAD CORP ) discloses an ink-jet printing process using charged droplets wherein the ink composition contains (a) a multifunctional unsaturated UV-curable compound, (b) a monofunctional unsaturated compound, (c) a reactive synergist, (d) a colorant, (e) an oil soluble salt for conductivity, (f) a photo-initiator, and (g) an organic polar solvent, preferably in a small amount.
However the behaviour and interaction of a UV-curable ink on a substantially non-absorbing ink-receiver was found to be quite complicated compared to a water- or a solvent-based ink on absorbent ink-receivers. In particular, a good and controlled spreading of the ink on the substrate is problematic.
EP 1199181 A (TOYO INK MANUFACTURING CO.) discloses a method for ink-jet printing on a surface of a substrate of a synthetic resin, said method comprising the steps of:                1. conducting a surface treatment to said surface so as to provide the surface with a specific surface free energy of 65-72 mJ/m2         2. providing an activation energy beam curable ink having a surface tension of 25-40 mN/m        3. discharging the ink onto the surface having the specific surface free energy with an ink-jet printing device thereby forming printed portions of said ink on the surface and        4. projecting an activation energy beam onto the printed portions.        
Surface tension and surface energy are equivalent parameters. The surface tension of a liquid is defined as the force acting on a unit length of the surface and is expressed in mN/m, whereas surface energy of a solid is the energy needed to create a unit area of interface and is expressed in mJ/m2. These dimensions are equivalent: mN/m×m/m=mJ/m2. For consistency in disclosing the present invention, the term surface energy of an ink will be used instead of the term surface tension of an ink. Hence, in step 2 of the method disclosed by EP 1199181 A (TOYO INK MANUFACTURING CO.) the surface tension of 25-40 mN/m is equivalent to a surface energy of 25-40 mJ/m2.
The method of EP 1199181 A (TOYO INK MANUFACTURING CO.) further appears to teach that the surface energy of the ink-receiver surface should be greater than the surface energy of the ink. Yet in the examples, although the surface energy of the four untreated synthetic resin substrates (ABS, PBT, PE and PS) was higher than the surface energy of the four different inks, a good ‘quality of image’ i.e. good spreading of the ink was not observed. EP 1199181 A (TOYO INK MANUFACTURING CO.) is also silent about the dispersive and polar components of surface energy of the ink and the substrate. The surface treatments used in the examples were corona treatment and plasma treatments.
JP 2003261799 (KONICA) discloses a method for jetting inks containing radical polymerization initiators, polymerizable monomers, and surface active agents onto preferably non-absorbing recording media and irradiating them with UV-light. An oriented polypropylene (OPP) sheet with a surface energy of 38 dyne/cm was printed using a set of black, yellow, magenta and cyan inks, having surface tensions from 24 to 30 mN/m, and containing 24 wt % of Kyarad™ DPCA (hexaacrylate), 23 wt % of Biscoat™ 335HP (tetraethylene glycol diacrylate), 41 wt % of light acrylate PO-A™ (phenoxyethyl acrylate), 5 wt % of Irgacure™ 369 (initiator) and 3 wt % of a vinyl monomer-type surface active agent.
WO 03074619 A (DOTRIX NV—SERICOL LTD) discloses a progressive dot printing ink-jet process comprising the steps of applying a first ink drop to a substrate and subsequently applying a second ink drop on to the first ink drop without intermediate solidification of the first ink drop, wherein the first and second ink drops have a different viscosity, surface tension or curing speed. It also claims a set of ink-jet inks suitable for use in a progressive dot printing ink-jet process comprising at least two inks having a different viscosity, surface tension or curing speed.
In the examples WO 03074619 A (DOTRIX NV—SERICOL LTD) discloses the use of a high-speed printer for printing UV-curable inks on a PVC substrate. This was performed in a ‘wet-on-wet printing’ process, wherein the first/subsequent ink drops are not cured, i.e. they are not irradiated prior to application of the next ink drop. In this way an increase in the ink spreading can be realized due to the increased volume of ink of the combined ink drops on the substrate. The WO 03074619 A (DOTRIX NV—SERICOL LTD) is also silent about the dispersive and polar components of surface energy of the ink and the substrate.
There is therefore a need to provide an ink-jet printing system suitable for high areal throughput printing (i.e. high area of ink-receiver printed per unit time, e.g. >100 m2/hour) with curable inks, which enables different types of ink-receivers to be used.